Plex Media Server Version
Make sure you are using the latest version of Plex Media Server. Earlier versions may have had issues with particular types of channels which were fixed with newer releases. Having the latest version of Plex ensures you have the benefit of more recent bugfixes. Sometimes when there is a major update, it may not automatically alert you or show up when you choose the “Check for Updates” option. So you may need to manually check the Plex website to make sure you have the latest version of the Server.
Related Page: Plex Downloads
Verify Content Availability
Make sure the video or content is available on the website. Plex channels can only play videos that are available and play on the website. Before reporting an issue with a particular channel not having specific content or errors playing a particular video, try accessing and playing it on the website first. Also, be aware that high demand content or accessing content during peak periods when a lot of people are trying to view that website's content can also cause issues due to limitations of the website's server and their available bandwidth.
Make sure the content is not region restricted. Many websites limit their content based on the IP address that is used to access the content. You'll want to make sure that you properly set your region to ensure you are only accessing channels that work in your region.
Related Page: Channels > Region Settings
Check the Forums
Check any recent posts to the Plex channel forums. You can find out if there is any known issues that have been reported and updates on when there may be a solution. If it is a channel you either downloaded from the Unsupported Appstore or directly from GitHub, it may have special instructions or restrictions. Check the readme file on Github. Also, these plugins usually have one main topic on the forum. The first post for these topics are where the developer posts any issues, instructions, or limitations of the plugin.
If you are having issues with many or all channels playing content, this is usually an indication of issues with your Plex Media Server or the App you are using to access the Server, not the channels themselves. You need to determine whether the issue is with your Plex App or Plex Media Server by trying to access the Server with another App. If you do not have multiple Apps, the Plex Web App can be accessed by choosing the Media Manager option from the Server on the computer it is running. This will help you determine the source of the problem. Often, reviewing the section of the Plex forums for your particular App or OS can provide troubleshooting or known limitations.
If it is a channel you downloaded directly from a GitHub repository, it could be old code or an abandoned projects that no longer works. You can check the GitHub repository site to see if the creator still maintains the channel bundle and try to contact the creator by submitting an issue. If the bundle has not been updated lately and/or has major issues listed that have not been addressed in a while, most likely it is an old and abandoned project that just will not work and it is better to just remove the channel from your system.